In this application we propose various complementary single molecule assays to investigate the mechanical translocation of individual E. coli RNA polymerase molecules during transcription elongation. Through these studies, we seek to determine the complex dynamics of RNA polymerase (RNAP) as it moves along the template and characterize the relationship between nucleotide addition and translocation as a first step to ultimately dissect the details of its mechanochemicalcycle at high resolution. Specifically,we propose to: 1. Characterize pausing and arrest behavior during continued, uninterrupted elongation using an integrated optical trapping/flow control video microscope currently operating in our laboratory. 2. Characterize the mechanochemical cycle of the motor. To this end, we will obtain force-velocitycurves for E. coli RNA polymerase under a variety of conditions. These curves will be analyzed using a molecular motor theory recently developed in our laboratory. 3. Develop a high resolution translocation assay to directly observe the dynamics of translocation of RNAP at single bp resolution. 4. Characterize the dynamics of translocation of RNAPagainst torsional stress by performing single molecule transcription assays with torsionally constrained DNA both in the presence and absence of E.coli gyrase. Through the single molecule studies describe here, it will be possible to follow individual molecular events that would otherwise be missed in the ensemble average ofbulk measurements. These events, involving dynamical changes in structure and function of the enzyme, lie at the heart of the process of regulation of gene expression. Finally, in the experiments described here, mechanical force will be used as a new controllable variable to characterize the process of mechanochemical transduction in this motor enzyme.